Powder coating compositions have become increasingly important because they give off very little or no volatile organic material to the environment when cured. Typically, any such emissions are limited to by-products of the curing reaction, such as blocking agents or volatile condensation by-products. Powder coatings have found use as both decorative coatings and protective coatings.
Topcoats for automotive and other industrial applications may be one-layer coatings, in which the color is generally uniform through the coating layer, or clearcoat-basecoat composite coatings, having a colored basecoat layer underlying a transparent clearcoat layer. Clearcoat-basecoat composite coatings, including “tri-coat” systems of clearcoat-interference coat-ground (or color) coat, are widely used in the coatings art and are notable for desirable gloss, depth of color, distinctness of image and/or special metallic effects. Composite systems are particularly utilized by the automotive industry to achieve a mirror-like, glossy finish with a high depth of image. All of the coating layers, including the underlying primer layer or layers, should be as smooth as possible to attain the best depth of image.
It is challenging to obtain such a high degree of smoothness using a powder coating. While solventborne and waterborne coatings may be formulated with materials with low glass transition temperatures, such materials are avoided for powder coatings so that the coating will not sinter. Further, organic solvents or cosolvents in the liquid coatings provide flow and leveling when the coating coalesces and cures. It is difficult to achieve the same appearance using a powder coating. Powder slurry coatings allow application using the same methods as liquid coatings, but, again, the liquid medium of a powder slurry coating does not solvate the dispersed powder and (to a large extent) does not plasticize or provide flow of the powder coating during the bake. Thus, an object of the invention is to provide a powder coating composition that produces a coating layer with exceptional smoothness.